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Trace element geothermometry and geospeedometry for cumulate rocks: Quantitative constraints on thermal and magmatic processes during igneous crust formation
  • Chenguang Sun
Chenguang Sun
Department of Earth, Environmental and Planetary Sciences, Rice University, Department of Earth, Environmental and Planetary Sciences, Rice University

Corresponding Author:csun@rice.edu

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Cumulate rocks record the magmatic and cooling processes during formation of Earth’s igneous crust. Extracting the information of these two processes from mineral records, however, is often convoluted by various extents of diffusive resetting during cooling subsequent to main stages of crystallization. Accordingly, for cumulate rocks at diffusive closure, the apparent “equilibrium” temperatures derived from geothermometers are generally lower than the crystallization temperatures. Using analytical or numerical models, geospeedometers can extract cooling rates from the closure temperatures (or profiles) but only if the initial temperatures are determined independently. Here I summarize the general framework of geothermometry and geospeedometry from a trace element perspective. The Mg and REE-based exchange geothermometers for mafic cumulate rocks are reviewed as examples of the geothermometer design. Based on the observed differential diffusive closures of Mg and REE in oceanic gabbros, I outline a general resolution to uniquely determine the initial crystallization temperature and cooling rate of a cumulate rock. The basic idea is further demonstrated using the recently developed Mg-REE coupled geospeedometer for mafic cumulate rocks. Finally, I use the Hess Deep gabbros as a case study to show that this two-element coupled geospeedometer is particularly useful to delineate the igneous accretion and cooling styles during crust formation. This two-element (or multi-element) coupled approach outlined here can also be readily extended for decoding comprehensive thermal histories of other petrological systems at various geological settings or other rocky planetary bodies.
31 May 2021Published in Geophysical Monograph Series on pages 19-43. 10.1002/9781119564485.ch2